A glucocorticoid dependent enhancer within the long terminal repeat (LTR) of Moloney murine sarcoma virus (MoMSV) has been identified that coresides with a hormone independent enhancer. The long term goal of this project is to examine within MoMSV and related C-type retroviruses, the relationship between distinct transcriptional regulatory elements, and uncover genetic strategies by which multiple regulatory properties are superimposed. Transient transfections will be performed into different cell types with in vitro generated MoMSV mutants to identify specific sequences that are necessary for hormone dependent and independent enhancers and reveal any cell type specificity of enhancer activity. We will also analyze glucocorticoid effects on the expression of endogenous C-type proviruses. Intact and hybrid MoMSV proviruses will be generated in various cell types by stable transfections, in order to test the effect of proviral context and examine the kinetics, cell type specificity and cellular heterogeneity of the glucocorticoid response of C-type retroviruses. In addition, the transcriptional effects of glucocorticoids on endogenous xenotropic proviruses will be studied and viral sequences reponsible for the hormone response identified by cDNA cloning. Finally, we will analyze the molecular basis for the differential activity of distinct MoMSV enhancers in undifferentiated embryonal carcinoma cells. Both transient and stable transfections will be performed with MoMSV DNA to examine both cis- and trans-acting mechanisms that influence glucocorticoid regulated expression.